Reliance on fossil fuels to power transportation and electricity networks is causing multiple and simultaneous energy crises. Coal, oil and natural gas all contribute to global warming and climate change, while creating geopolitical instability and energy insecurity. Recently, many developed and developing countries have turned to biofuels to solve those problems.
Unfortunately, obtaining biofuels from plant sources presents its own dilemmas. Any acre seeded for energy crops is likely to supplant an acre of food crops, which in turn causes a rise in the cost of basic food staples. Agriculture utilizes a significant amount of energy, much of which is supplied in the form of fossil fuels, thus lowering the overall carbon balance of biofuel crops. In addition, the yields of most energy crops are quite small. An acre of corn produces only about 350 gallons of ethanol per year, while an acre of soy can produce about 50 gallons of biodiesel per year. Finally, use of agriculturally marginal land to grow energy crops is limited by the lack of sufficient water available to support plant growth.
A biofuel that can be grown productively on otherwise unusable land with minimal energy input is needed to convert to a sustainable energy infrastructure. No traditional land crop presents a complete solution. However, microalgae hold the promise to address each of the limiting factors of biofuels. Closed system photobioreactors can be sited on otherwise unusable land, limiting competition with food crops and minimizing water losses due to evaporation. Yields of biofuels from algae can potentially exceed the yields of soil crops by orders of magnitude (see, e.g., Natl. Renewable Energy Laboratory, “A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae,” NREL/TP-580-24190, July 1998). Algal cell growth can quickly replace extracted material, promising a year-round harvest. As an added bonus, the microalgae can utilize the carbon dioxide-rich flue gases of fossil-fuel power plants and other industrial exhaust gases, thus lowering the quantity of greenhouse gases expelled into the atmosphere from power plants, breweries, wineries and the like.
As of yet, there are no commercial-scale microalgae farms built for the purpose of producing biofuels. Traditional industrial photobioreactors have not yet been able to repeat the high cell densities achieved in photobioreactors on the laboratory scale. In order to produce energy crops from microalgae on a competitive basis with fossil petroleum, extremely dense cultures must be grown inside the photobioreactors, which take maximum advantage of available solar light. In addition, the materials used to build these photobioreactors must be made cost effective. Traditional closed photobioreactors, made of expensive glass and steel components and complex pumps and flow distribution systems, will not achieve biocrude cost targets that are competitive with fossil fuels. Such systems are also poorly scalable from lab bench units to commercial scale production (e.g., Grima et al., J. Applied Phycology 12:355-68, 2000). A need exists in the field for economical, efficient closed system photobioreactors that are capable of growing high density algal cultures, designed to optimize utilization of solar light, and to produce biofuels at a price point that is competitive with fossil fuels.